Therapeutic hypothermia is being used more frequently in the pediatric intensive care unit, and is being studied in the setting of pediatric cardiac arrest. Following cardiac arrest, multiple organ dysfunction syndrome (MODS), especially renal and hepatic dysfunction, is common and affects the metabolism and excretion of drugs. In addition, very little is known about the impact of hypothermia on a child's ability to metabolize medications. Dose adjustments may be required in the setting of hypothermia to avoid under-dosing and over- dosing of medications. Improper dosing and drug accumulation of sedatives and opiates can worsen existing neurologic, circulatory and respiratory failure. The measurement of the actual drug and metabolite concentrations in the body (pharmacokinetics) provides information on how a child metabolizes medications. In addition, variability in these concentrations after the administration of equal doses to different children may result from genetically driven differences in drug metabolizing systems (pharmacogenetics). Finally, these genetic differences may respond differently to hypothermia. The parent R01, Therapeutic Hypothermia After Pediatric Cardiac Arrest, comparing the efficacy of therapeutic normothermia vs. hypothermia to improve neurologic survival provides a unique opportunity to study the impact of organ failure, pharmacogenetics and hypothermia on metabolism, clearance and drug disposition. Thus, our overarching hypothesis is that morphine and midazolam disposition will be affected by temperature management even when accounting for potentially confounding quantifiable factors of organ dysfunction and genetic differences. The objectives of this ancillary R01 application, Hypothermia's Impact on Pharmacology (HIP) are to 1) determine the impact of organ dysfunction following cardiac arrest on the pharmacokinetics of morphine and midazolam, 2) identify the impact of hypothermia on the pharmacokinetics of morphine and midazolam and 3) identify the influence of polymorphisms on drug metabolizing systems on the response to hypothermia in children who have experienced cardiac arrest. Sophisticated modeling and simulation techniques will be utilized to examine the highly dynamic changes in physiology associated with critical illness, drug disposition, pharmacogenetics and temperature modulation. The models created using this approach will be implemented to optimize the prospective treatment of these critically ill children.